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The Evolution of Quasars with Cosmic Time

While the luminosity and mass distributions of quasars
has evolved dramatically with cosmic time, the physical properties of quasars
of a given luminosity are remarkably independent of redshift. I will describe recent results on the spectra
of luminous quasars, the dark matter halos in which they sit, and the
intergalactic medium of their host galaxies, that are essentially
indistinguishable from moderate redshifts to z>6.

The one property apparently unique to the
highest-redshift quasars is that some small fraction show evidence for having
very little infrared excess from hot dust.
Dust obscuration is another theme in quasar studies; an appreciable
fraction of the growth of black holes may be hidden at optical wavelengths by
dust. I will describe searches for
obscured quasars at high redshift and low, and studies of their demographics
and physical properties.